To investigate the distribution of next neighbors of human conephotoreceptors in images acquired in vivo with an adaptive opticsscanning laser ophthalmoscope (AO-SLO).

Methods:

An AO-SLO instrument operating at 40 fps (imaging area ~300x 300µm2) with closed loop adaptive optics configurationis used to record images of the human cone mosaic. To increaseimage quality 40 images are registered to each other and averagedprior to data analysis. Custom software is used to detect eachcone. To obtain the number of next neighbors, the distancesof each cone to its closest cones are measured. The algorithmregards cones that are within ±30% of the mean distanceof the closest seven cones as next neighbors. Finally, the intensitydistribution of cones with different numbers of next neighborsis measured.

Results:

The number of next neighbor cones could be determined in a healthyvolunteer at different locations (1.2° to 7.2° eccentricityfrom the fovea) on the retina. At 1.2° eccentricity fromthe fovea 57% of the cones are surrounded by 5 next neighbors(intensity 100.6% of all cone average), 33% have 6 next neighbors(intensity 99.6%) and 10% have 7 next neighbors(intensity 96.7%).This percentage is changed at 7.2 degrees to 40% with 5 nextneighbors (intensity 88.2%), 37% with 6 next neighbors (intensity105.8%) and 23% with 7 neighbors (intensity 111.6%). The distributionof cones with irregular number of next neighbors appears notto be random which corresponds to previously reported observationsthat have been obtained in vitro.
Fig.1. AO-SLO image of the cone mosaic at 2° eccentricity(left). Number of next neighbors measured for each cone at thiseccentricity (right). Green indicates 5 neighbors, red indicates6 neighbors and blue indicates 7 neighbors.

The number of next neighbors could be determined in AO-SLO imagesof the human retina. The distribution of cones with irregularnumber of next neighbors is not random and changes with eccentricityfrom the fovea.